Abstract
Since the days of the beginning of our species, the process of trying to understand the very nature of the stars, the Moon, the planets and the Earth itself is still a work in progress. Only in the last five decades, we have acquired the capability to observe the Earth from outside, to land on another planet or to detect the radiation emitted by an exploding star in a region of the space far beyond any visible object in the sky and in a frequency range totally outside the boundaries of our senses. Our planet is part of a planetary system rotating around a star, the Sun. Earth and two neighboring planets in this system, Venus and Mars, share the same birth and have similar masses, composition and general characteristics, at least when regarded at a global scale. However, Venus, Mars and the Earth have followed very different planetary evolutionary paths over the about 4.5 billion of years of the history of the Solar System. Having started off from the same protoplanetary disk, and being positioned not at so different distances from our star, they now have surface temperatures and atmospheric compositions widely different. Venus features an atmosphere about 100 times thicker than our own, mostly made of CO2 and water vapor, the greenhouse gases par excellence. It thus suffers from a disastrous runaway green house effect, leading to surface temperatures close to 500 °C. Mars, in contrast, today has a very thin atmosphere, about 100th of Earth’s, and surface temperatures that are very cold even by Siberian standards. What happened exactly to this triplet of “terrestrial” planets to create differences today? Space planetology tries hard to give an answer to this question and, in passing, to understand how, on Earth, we can avoid following similar or equally catastrophic paths. We have also explored the fourth, and last, atmosphere around a solid body in our planetary system: Titan, the biggest of Saturn satellites. It is made of water ice, methane and other hydrocarbons, in gaseous, liquid and solid forms, yielding many interesting species of complex, organic molecules to populate what is possibly a prebiotic world. But Anthropocene is creeping up: it expands from our Earth into our solar system; we have deposited tons of earthly materials on the Moon and only slightly less on Mars, Venus and on some other planetary bodies, including asteroids and, in the very near future, a comet. It is very unlikely that all of the terrestrial materials scattered into the solar system had been properly sterilized. This is especially true for the first generation of probes that landed on the Moon or Mars before that a planetary protection policy was issued and agreed upon by all major space agencies.